In addition to the printing function, the image forming system performs the function of enhancing the efficiency of clerical work by using a finishing device provided to perform various types of finishing, in recent years. Finishing includes punching, stapling, inserter processing for insertion of a cover sheet or insert sheet, folding processing such as center folding or triple folding, and bookbinding, for example.
One of the bookbinding processes is a step of ring binding, wherein a ring binder disclosed in for example, Unexamined Japanese Patent Application Publication No. 2008—094081 is fitted into a plurality of holes formed on a bundle of sheets. When the ring is closed, a bundle of sheets is ring-bound, whereby a booklet as shown in FIG. 6 is produced.
Matching between the thickness of a bundle of sheets and size of the ring binder is important in ring binding. If a thin bundle of sheets is equipped with a large ring binder, the product looks less attractive. If a thick bundle of sheets is provided with a small ring binder, the booklet will be difficult to open, and usability will be impaired. If a small ring binder is fitted forcibly, a ring binding device may be damaged.
Unexamined Japanese Patent Application Publication No. 2008—094081 discloses a technique of ring binding, wherein a sheet thickness sensor for measuring the thickness of a bundle of sheets is provided. The ring binder suited for the thickness having been measured is selected to perform ring-binding. Further, according to the second embodiment of the Unexamined Japanese Patent Application Publication No. 2008—094081, in the case of a plurality of copies of ring binding, the size of the ring binder determined based on the sheet thickness of the first copy is memorized. When the second copy or copies thereafter are to be printed again, ring binding operation is performed by the ring binder of the memorized size, if the sheet thickness is kept within a prescribed range. This allows the ring binder of the same size to be used when the same image data is ring bound.
The technique disclosed in Unexamined Japanese Patent Application Publication No. 2008—94081 uses a ring binder of the same size to bind booklets which are to be created according to one and the same job. The sheet thickness is measured for each of a plurality of copies of a bundle of sheets. Although Unexamined Japanese Patent Application Publication No. 2008—094081 does not disclose the method for measuring the thickness, it is possible to consider a method wherein a movable gauge is moved to keep a bundle of sheets sandwiched in-between and the moving distance of the movable gauge is measured. In that method, however, measurement of thickness requires a prescribed time. If the thickness is measured for a plurality of all copies, productivity will be reduced.
In view of the problems described above, an object of the present invention is to provide an image forming system characterized by a high degree of productivity and attractive outward appearance wherein booklets produced from one and the same job are bound by a ring binder of the same size.